The present invention relates generally to command, control and communications systems, and more particularly to a technique for the acquisition, synchronization, and recovery of coherently coded and combined spread spectrum symbol formats.
With the continuing development of sophisticated command, control, and communication data processing systems, spread spectrum communication techniques have drawn particular attention because of a number of advantages they offer over more conventional and limited bandwidth modulation schemes. One advantage is the capability of enabling the communication link to exhibit a robustness against jamming or natural interfering signals which are not correlated with the particular spreading waveform. These interference signals may include jamming, randomly distributed natural events, or other users of the same spectrum. A further advantage is that a signal-to-noise improvement may be obtained by systems which employ a plurality of codes (symbol alphabet) by transmitting a sequence of spread symbols whose energy distance has been maximized and equalized to enhance the decision thresholds as opposed to using an uncoded signal. In addition, enhanced time resolution may be obtained with the increased bandwidth.
The trend towards "Intelligent Jamming" mandates future tactical communications systems possess Electronic Counter Counter Measures (ECCM) responsive signal formats. The ECCM response to interference must be real time and adaptive to maximize the probability of successfully receiving a message. In addition, the "Adapted" transmission must be burst transmissions to enhance the Low Probability of Intercept (LPI). Consistent with burst transmission is the requirement for rapid data synchronization without repeat transmissions.
The determination of the signal to noise ratio at the output of the correlator to a communication system is considered to be fundamental to any ECCM adaption. The objective of the adaptation is the maintenance of the minimum information bandwidth (BWI). The signal-to-noise ratio of the correlator output (S/N)out will specify a Bit Error Rate (BER) which must be equal to or less than the BER required to maintain the BWI. Other factors (i.e., error correcting coding/interleaver gains, decoder efficiency, etc.) will determine this maximum BER for successful transmission With proper knowledge of (S/N)out, a successful adaptation can be achieved.
United States patents of interest include U.S. Pat. No. 4,730,340 to Frazier, Jr., which discloses a communication system which includes the acquisition, synchronization and recovery of coherently coded and combined spread spectrum symbol formats. In the receiver of the patent an incoming unknown symbol sequence capable of being acquired is applied in parallel to all cells of a matched filter correlator array. Horwitz in U.S. Pat. No. 4,644,523, improves the signal-to-noise ratio in a spread spectrum system by using a plurality of transmitters synchronized by a common clock and spread by a common bipolar pseudo-random code. Bjornholt et al, in U.S. Pat. No. 4,447,907, use multiple code generators and multiple mixers in a spread spectrum system. Multiple continuous spread spectrum signals are taught by Ward et al in U.S. Pat. No. 4,164,628 Lambert U.S. Pat. No. 4,320,513 describes a spread spectrum system having a circuit for the production of a number of different codes.
Historically, coherent detection of a non-spread digital signal such as a binary amplitude modulated (AM) signal requires the receiver to possess knowledge of the phase of the received signal carrier. To be coherent, the receiver must have or generate a carrier whose frequency and phase match the incoming carrier. In practice, the receiver does not possess this knowledge but rather extracts the information from the received signal from which a carrier can be generated.
Definitions of some terms are:
1. Modulation: Alteration of the frequency, phase or amplitude of a wave in accordance with a signal.
2. Symbol: A discrete pattern displayed or transmitted in order to convey information.
3. Spread Spectrum Artificially broadens the message spectrum prior to transmission.
4. Chip: Time duration of one alteration of the spreading code, a "1" or a "0".